Conventionally, boilers provided in thermal power stations and a variety of large-scale boilers employing a fuel such as petroleum, coal, or fuel gas, waste incinerators, and similar apparatuses have been equipped with a flue gas NOx removal apparatus which contains a plurality of NOx removal catalyst layers.
The above employed NOx removal catalysts assume the form of honeycomb, plate, etc. During use, the catalytic performance of the catalysts is problematically deteriorated with elapse of time as a result of deposition, on the surface of the catalyst, of a substance which deteriorates the catalytic performance (hereinafter referred to as deteriorating substance) or through migration of the dissolved deteriorating substance into the catalyst.
Conventionally, the performance of the NOx removal catalysts has been managed by measuring NOx concentration and unreacted NH3 concentration on the inlet and outlet sides of respective catalysts. When a drop in total performance of a catalyst system is confirmed, old catalysts are replaced with new catalysts or regenerated catalysts in order of use age, and such replacement is carried out periodically.
Generally, NOx removal catalysts are very expensive. Thus, there has been proposed one approach for prolonging the service life of the NOx removal catalysts to as long a duration as possible by assessing the performance of each unit catalyst (Japanese Patent Publication (kokoku) No. 7-47108).
However, the aforementioned catalyst managing method has a drawback. According to the method, NOx concentration and unreacted NH3 concentration of each catalyst layer are determined, and percent NOx removal and percent contribution of each catalyst layer are calculated from the determined NOx concentration. On the basis of these values, performance-deteriorated catalysts are replaced with new catalysts in order of degree of deterioration. In this case, when the catalytic performance is evaluated by the percent contribution calculated on the basis of the NOx concentration, the catalyst layer(s) having actually deteriorated performance cannot be detected correctly.
In view of the foregoing, an object of the present invention is to provide an NOx removal catalyst management unit for use with an NOx removal apparatus, the management unit detecting an NOx removal catalyst layer that is actually deteriorated, whereby the deteriorated NOx removal catalyst layer can be effectively replaced by a new catalyst layer. Another object of the invention is to provide a method for managing the NOx removal catalyst.